The present invention relates to temperature measurement and, more particularly, to a device and method for monitoring the condition of a thermocouple.
Thermocouples are a type of temperature sensor used in a wide variety of applications. Customarily, a thermocouple is comprised of two dissimilar elements, referred to as thermoelements, which are joined at a bimetal junction. The principle on which a thermocouple operates is that a voltage or electromotive force (xe2x80x9cEMFxe2x80x9d) is created at the bimetal junction of the two dissimilar elements. The bimetal junction voltage corresponds to a temperature.
Over time, thermoelements may suffer changes in composition due to operational stresses. Such changes may result in the thermocouple not providing a true temperature. Compositional changes are referred to generally by the term degradation. Degradation can result in the thermoelement providing inaccurate temperature measurements.
When a thermoelement has suffered degradation, an operator of a temperature-dependent process may be able to recognize that degradation has occurred because the operator may intuitively know that the temperature reading being supplied by the thermoelement is outside the normal range of temperatures at which the process operates. However, in some instances, the operator may not be able to recognize that a thermoelement has degraded. Further, an operator is generally unable to recognize the initial stages of degradation in a thermoelement. In such a situation, a temperature-dependent process may still be within an acceptable range of operating temperatures, but not at an optimum operating temperature, or the process may be operating outside an acceptable range of operating temperatures.
The present invention is directed to a device and method for monitoring one or more thermoelements in a thermocouple, enabling evaluation of the condition of the thermoelements while the thermocouple is in operation. The device and method, in some embodiments, is capable of detecting the initial stages of degradation of one or more thermoelements in a thermocouple.
In one embodiment, a thermocouple consisting of two or more thermoelements joined at a common junction point has a like number of diagnostic elements connected to the common junction point. In an alternative embodiment, the thermocouple has a single diagnostic element selectively coupled with one of the thermoelements of the thermocouple. In either embodiment, when connected with the thermoelement, the diagnostic element and thermoelement form a measurement loop around which a resistance or impedance measurement can be made.
In selecting a diagnostic element one consideration is that the diagnostic element be relatively more stable, i.e., less prone to undergo a change in resistance as a result of operational stresses, than the thermoelement or thermoelements with which the diagnostic element is being paired. However, it is to be understood that a diagnostic element which is less stable than the thermoelement or thermoelements with which it is being paired may also be used in the present invention. In operation, an initial loop resistance is measured and established as a reference or baseline. The loop is then monitored while the thermocouple is in operation for changes in loop resistance. Normally, changes in the loop resistance are an accurate indicator of thermoelement degradation. In the various embodiments, the thermoelement/diagnostic element loops are coupled to measurement electronics. The measurement electronics periodically measure the resistance or impedance of each of the various thermoelement/diagnostic element loops for changes in the loop resistance or impedance at a given temperature.
The present invention is capable of offering one or more advantages. For example, a device in accordance with an embodiment of the present invention can be configured to allow an operator to monitor the thermoelements of a thermocouple sensor assembly for changes in resistance which are indicative of thermoelement degradation. A further advantage of the present invention is that a device in accordance with an embodiment of the present invention allows the thermoelements to be monitored while the sensor is in operation. Also, a device in accordance with an embodiment of the present invention can quantify the degree of degradation which has occurred in the various thermoelements comprising the thermocouple sensor assembly.
Other features and advantages of the invention are apparent from the following detailed description of the invention and the claims.